There's tension around every election cycle between announcing your candidacy early and waiting until the right moment. From a campaigning/messaging standpoint the right moment is almost always to wait as long as possible. It gives the media less time to pick you apart, less time for popular culture to turn on you, less time to make mistakes, less scrutiny. But it's often necessary to get in the race early if your brand isn't strong, and because you want to lock up fundraising, volunteers, super pacs, etc.
What's unique about this candidacy, aside from an incumbent President stepping down, is Harris didn't have to do the primary circuit, didn't have to rebuff attacks from her own party, didn't have to negotiate policies with party powerbrokers, didn't have to slowly build a campaign machine. She inherited Biden's candidacy, inherited his fundraising apparatus, and then capitalized with momentum and energy and organization.
In a way it's a condemnation of the drawn-out primary system which is meant to means-test candidates but in actuality can damage the eventual front-runner. Hillary vs Obama and Hillary vs Bernie are two recent examples of the Democrats being unable or unwilling to align behind a standard-bearer and behind a unified message, and Trump vs Cruz and Trump vs Hailey on the GOP side as well. In both cases the candidate has to navigate a contentious battle with their own party before the national campaign even begins, so they come out damaged and attacked. Harris has this unique opportunity where she's been handed the keys to the party without having to battle it out, and it might be showing us the real advantage of avoiding two years of party in-fighting.
I have such a clear memory of Elizabeth Warren announcing her candidacy long before anyone else in the party, and her approach was to talk about specific legislation she believed she could get passed, budget proposals and government operations. And with no real opponent everyone picked apart her ideas. It didn't matter that they didn't propose any ideas of their own, she gave them something real to evaluate and criticize, and by the time the campaigning began in earnest she had already flamed out.